1. Field of the Invention.
This invention relates to the field of stereophonic FM broadcasting, and more particularly to a method and circuitry for precluding or minimizing to a negligible level the degree of overshoot of a lowpass filter network beyond a predetermined signal excursion limit.
2. Description of the Prior Art.
A preliminary patentability search was conducted in the following classes and sub-classes with respect to the subject invention:
Class 307--sub-classes 543; 546; 547 and 556. PA1 Class 328--sub-classes 169; 170; 171 and 175. PA1 Class 330--sub-classes 126 and 151; and PA1 Class 381--sub-classes 13; 14; 94 and 106 PA1 U.S. Pat. Nos. 3,651,339; 3,968,384; 3,986,049; 4,110,692; 4,103,243; 4,208,548; 4,241,266; 4,441,199.
As a result of the search in this area, eight (8) U.S. patents as follows were found:
In addition, three additional U.S. patents have come to my attention and have been considered, and are identified as U.S. Pat. Nos. 4,134,074; 4,383,229; and 4,460,871
Of the eight patents listed above that were found as a result of the search, none appear to be relevant with respect to the invention described herein. Of the three patents listed in the immediately preceeding paragraph, only U.S. Pat. Nos. 4,134,074 and 4,460,871 appear to have any pertinency.
U.S. Pat. No. 4,134,074 to Hershberger directly addresses the problem of lowpass filter overshoots in stereophonic FM broadcasting. However, the method and apparatus taught by Hershberger are different in structure and function from the method and apparatus forming the subject matter of the instant invention. In Hershberger, two nearly identical lowpass filters are placed in the signal path. The amplitude overshoots of the first filter are isolated from and recombined with the signal in such a way that they cancel not only the first filter overshoots, but predict and inhibit overshoots in the second filter as well. Thus, the Hershberger U.S. Pat. No. 4,134,074 circuit satisfies the requirement with full consideration to the bandwith and amplitude constraints, but at the expense of a second, otherwise redundant lowpass filter circuit. In addition, another disadvantage of the Hershberger circuit, is that it calls for a constant "group delay" characteristic in the second filter, no small matter in filters of the higher order, more complex designs common to FM broadcasting.
U.S. Pat. No. 4,460,871, issued approximately five and a half years after Hershberger U.S. Pat. No. 4,134,074, constitutes a refinement of the concept disclosed by Hershberger. In Orban's U.S. Pat. No. 4,460,871 circuit, the primary lowpass filter is permitted to overshoot in the expected manner. Overshoots are again isolated and reintroduced to the signal path in a manner which opposes and thus cancels the amplitude over excursions. Rather than pass the entire "corrected" signal through a second lowpass filter as is done in Hershberger, only the isolated overshoots are filtered before reintroduction. Thus, in addition to an otherwise redundant low-pass filter, Orban's U.S. Pat. No. 4,460,871 calls for signal equalization, a second "safety" clipper and a third, yet simple, lowpass filter element.
Thus, with respect to U.S. Pat. Nos. 4,134,074 and 4,460,871, each of these patents discloses at least one low-pass filter or other means of program bandwith restriction which, itself, exhibits normal amplitude overshoot characteristics.
Additionally, in each of these patents some means is required to isolate and control the amplitude overshoots of the first lowpass element, but at the expense of harmonic generation.
Still further, in each of these patents, a second low pass filter element is employed to suppress signal harmonics generated by overshoot correction circuitry.
Accordingly, one of the primary objects of the present invention is to eliminate much of the circuitry required by these patents and to provide overshoot protection by a much simpler circuit utilizing fewer components.
Still another object is the provision of an overshoot protection circuit which eliminates the need for multiple low-pass filter assemblies.
A still further object is the provision of a filter overshoot control circuitry that may be used with almost any variety of existing lowpass filter designs including those already in place in commercial broadcasting equipment.
Yet another object of the invention attendant to the factor of simplicity in design, is the provision of filter overshoot control circuitry which presents a more direct signal path to the audio program, lessening the chance for sonic degradation.
Since the filter overshoot control circuitry of this invention may be utilized with existing lowpass filter designs that are already in place in commercial broadcast equipment, another object of the invention is to lessen considerably the cost of implementing filter overshoot control circuitry in already in place commercial broadcast equipment.
The invention possesses other objects and features of advantage, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood however that the invention is not limited to the embodiment illustrated and described since it may be embodied in various forms within the scope of the appended claims.